Heavy duty tire

ABSTRACT

When a tread thickness from a second belt ply 7B is defined as T, a minimum position Qt in which the tread thickness T becomes a minimum value Tmin is provided in a region Y which is away from a tire equator C by a distance of 0.5 to 0.7 times of a tread grounding half width WT/2. A ratio Tmin/Tc between said minimum value Tmin and the tread thickness Tc at the position of the tire equator C is 0.92 to 0.97, and a ratio Tb/Tc between a tread thickness Tb at a position of an outer end of the second belt ply and said tread thickness Tc is 0.95 to 1.10. When a tread thickness from a carcass 6 is defined as K, a minimum position Qk in which a tread thickness K becomes a minimum value Kmin is provided in said region Y. A ratio Kmin/Kc between said minimum value Kmin and a tread thickness Kc at a position of the tire equator C is 0.97 to 0.998, and a ratio Kb/Kc between the tread thickness Kb at a position of the outer end of the second belt ply and said tread thickness Kc is 1.2 to 1.5.

FIELD OF THE INVENTION

[0001] The present invention relates to a heavy duty tire which uniformswear by specifying a tread thickness from a second belt ply and carcass.

BACKGROUND ART

[0002] In the case of a heavy duty tire for example, its tread outlineshape a is formed into a single arc shape in a vulcanizing metal mold asschematically shown in FIG. 3.

[0003] In such a tire, however, in a standard internal pressure state inwhich the tire is assembled into a rim and a standard internal pressureis charged into the tire, there is a tendency that a tread surfaceswells radially outward in a region Y which is separated from a tireequator by a distance of 0.5 to 0.7 times of a tread half width. Forthis reason, a difference in circumferential lengths between a swellingportion b and a tread grounding end e is increased, slip between thetread surface on the side of the tread grounding end and a road surfaceis generated, and uneven wear such as so-called shoulder wear is proneto be generated.

[0004] In order to restrain the shoulder wear, Japanese PatentApplication Laid-open No. H7-164823 and the like disclose a technique inwhich the outline of the tread in the vulcanizing metal mold is formedinto a double radius shape in which a portion of the tread on the sideof the tread grounding end is an arc having a greater radius ofcurvature than a portion of the tread on the side of the tire equator,thereby bringing the outline of the tread in the standard internalpressure state closer to the single arc to restrain the shoulder wear.

[0005] Such a technique can restrain the shoulder wear in some degreebut there is a problem that new shoulder wear is produced in the regionY. Especially when a circumferential groove is formed in the region Y,there is a tendency that so-called railway wears in which the grooveside edge of the circumferential groove wears are seriously generated.

[0006] It is conventionally known that there is a correlation between agrounding surface shape and an uneven wear of a tire, and a slip isgenerated in a portion of the tire in which the grounding length isshortened and wear proceeds fast. Thereupon, the present inventorresearched that the grounding surface shape was improved to suppress theuneven wear such as shoulder wear and railway wear, thereby equalizingthe wear.

[0007] As a result, the inventor found that the grounding surface shapehas a correlation between a tread thickness from a second belt ply to atread surface in a belt layer and a tread thickness from a carcass tothe tread surface, and if each of the tread thickness is limited, thegrounding surface shape could be optimized.

[0008] That is, it is an object of the invention to provide a heavy dutytire capable of improving the grounding surface shape and equalizing thewear based on an idea that the tread thickness from the second belt plyand the tread thickness from the carcass are limited.

DISCLOSURE OF THE INVENTION

[0009] An invention described in claim 1 of this application provides aheavy duty tire comprising a carcass extending from a tread portion to abead core of a bead portion through a sidewall portion, and a belt layerhaving a first belt ply arranged inside the tread portion and outsidethe carcass and a second belt ply outside the first belt ply, wherein

[0010] the second belt ply is curved in a form of projection of a singlearc having a radius of curvature Rb of 450 to 700 mm,

[0011] when a tread thickness between an outline of a tread surface andthe second belt ply is defined as T, a tread thickness minimum positionQt in which the tread thickness becomes a minimum value Tmin is providedin a region Y which is away from a tire equator C by a distance of 0.5to 0.7 times of a tread grounding half width, a ratio Tmin/Tc betweenthe minimum value Tmin and a tread thickness Tc at the position of thetire equator C is 0.92 to 0.97, and a ratio Tb/Tc between a treadthickness Tb at a position of an outer end of the second belt ply andthe tread thickness Tc is 0.95 to 1.10,

[0012] when a tread thickness between the outline of the tread surfaceand the carcass is defined as K, a tread thickness minimum position Qkin which the tread thickness K becomes a minimum value Kmin is providedin the region Y, a ratio Kmin/Kc between the minimum value Kmin and atread thickness Kc at a position of the tire equator C is 0.97 to 0.998,and a ratio Kb/Kc between the tread thickness Kb at a position of theouter end of the second belt ply and the tread thickness Kc is 1.2 to1.5.

[0013] In this specification, the term “tread grounding half width”means a distance between a tire equator C and a tire axially outer end(tread grounding end E) of a tread grounding surface which grounds whena standard load is applied to the tire in its standard internal pressurestate in which the tire is assembled into a rim and a standard internalpressure is charged into the tire.

[0014] The term “standard rim” is a rim whose specification aredetermined for each tire in the specification system including thespecification based on the tire. For example,

[0015] in the case of JATMA, a size of rim having a rim width narrowerthan a standard rim is “rim having a width which is one rank narrowerthan the standard rim”, and a size of a rim whose width narrower thanthe standard rim is not set means “standard rim”,

[0016] in the case of TRA, a size of rim having a rim width narrowerthan “Design Rim” is “rim having a width which is one rank narrower than“Design Rim”, and a size of a rim whose width narrower than the standardrim is not set means “Design Rim”, and

[0017] In the case of ETRTO, a size of rim having a rim width narrowerthan “Measuring Rim” is “rim having a width which is one rank narrowerthan “Measuring Rim”, and a size of a rim whose width narrower than thestandard rim is not set means “Measuring Rim”

[0018] Further, the term “standard internal pressure” is an air pressurewhose specifications are determined for each tire, and the standardinternal pressure is a maximum air pressure in the case of JATMA, thestandard internal pressure is a maximum value described in “TIRE LOADLIMITS AT VARIOUS COLD INFLATION PRESSURE” in the case of TRA, and thestandard internal pressure is an “INFLATION PRESSURE” in the case ofETRTO. If the tire is for a passenger car, the standard internalpressure is 180 kPa. Further, the “standard internal pressure” is airpressure determined by the specifications for each tire, and thestandard internal pressure is maximum load ability in the case of theJATMA, and the standard internal pressure is a maximum value describedin a table “TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES” in thecase of the TRA, and the and the standard internal pressure is “LOADCAPACITY” in the case of the ETRTO.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a sectional view of a tire according to an embodiment ofthe present invention.

[0020]FIG. 2 is an enlarged sectional view of a tread portion.

[0021]FIG. 3 is a diagram showing a tread outline shape in aconventional tire.

BEST MODE FOR CARRYING OUT THE INVENTION

[0022] An embodiment of the present invention will be explained based onthe illustrated example.

[0023]FIG. 1 is a sectional view of a heavy duty tire for a truck, a busor the like according to the present invention, and FIG. 2 is anenlarged view of a tread portion of the tire.

[0024] In FIG. 1, a pneumatic tire 1 includes a carcass 6 extending froma tread portion 2 to a bead core 5 of a bead portion 4 through asidewall portion 3, and a belt layer 7 disposed inside the tread portion2 and outside the carcass 6.

[0025] The carcass 6 is formed of one or more (one, in this example) ofcarcass ply 6A in which carcass cords are arranged at an angle of 70 to90° with respect to a circumferential direction of the tire. Metal cordssuch as steel are preferable as the carcass cords.

[0026] The carcass ply 6A includes ply bodies 6 a and 6 a extendingbetween the bead cores 5 and 5, and folded-back portions 6 b and 6 bfolded back and retained around the bead cores 5 and 5. A bead apexrubber 8 extending in a taper manner from the bead core 5 radiallyoutward is disposed between the ply main body 6 a and the folded-backportion 6 b. The bead apex rubber 8 reinforces a portion of the tirefrom the bead portion 4 to the sidewall portion 3.

[0027] The belt layer 7 is formed of three or more belt plies usingmetal cords as the belt cords. In this example, steel cords have fourplies, i.e., a first belt ply 7A which is arranged at an angle of 60±15°with respect to the circumferential direction of the tire and disposedat innermost portion in the radial direction, and the second to thefourth belt plies 7B, 7C and 7D arranged at small angle of 10 to 35°with respect to the circumferential direction of the tire.

[0028] In this belt layer 7, the ply width in the tire axial directionof the first belt ply 7A is smaller than the ply width of the secondbelt ply 7B and is substantially equal to the ply width of the thirdbelt ply 7C, the ply width WB of the second belt ply 7B which has themaximum width is set to 0.80 to 0.95 times of the tread grounding widthWT, thereby reinforcing substantially the entire width of the treadportion 2 with hooping effect, and enhancing the tread rigidity. Thefourth belt ply 7D having the narrowest width functions as a breakerwhich protects the first to third belt plies 7A to 7C and carcass 6 frombeing injured outside.

[0029] Then, on the tread portion 2, the tire 1 is provided with a treadpattern having two or more circumferential main grooves G whichcontinuously extend in the circumferential direction. Thecircumferential main groove G has a width of 3 mm or more, and extendsin the circumferential direction in a straight or zigzag form.

[0030] In this example, the circumferential main groove G comprises fourgrooves, i.e., inner circumferential main grooves Gi on opposite sidesof the tire equator C, and outer circumferential main grooves Go on theouter sides. The outer circumferential main grooves Go constitute theoutermost shoulder groove Gs in the tire axial direction.

[0031] The groove center line N of the shoulder groove Gs passes througha region Y which is away from the tire equator C by a distance of 0.5 to0.7 times of the tread grounding width WT/2. With this structure, thetread portion 2 is divided into a tread center portion Jc at inner sideof the shoulder groove Gs and a tread shoulder portion Js at outer sideof the shoulder groove Gs. When the shoulder groove Gs is a zigzaggroove, a center of the zigzag amplitude is defined as the groove centerline N.

[0032] In this embodiment, in order to equalize the wear in the tire 1,a tread thickness T between an outline S (tread outline S, hereinafter)of the tread surface and the second belt ply 7B, and a tread thickness Kfrom the tread outline S and the carcass 6 are specified as follows:

[0033] More specifically, as shown in FIG. 2,

[0034] (1) in the tread thickness T,

[0035] a tread thickness minimum position Qt in which the treadthickness T becomes a minimum value Tmin is provided in the region Y,

[0036] a ratio Tmin/Tc between the minimum value Tmin and a treadthickness Tc at a position of the tire equator C is set to 0.92 to 0.97,

[0037] a ratio Tb/Tc between the tread thickness Tb and the treadthickness Tc at a position on an outer end of the second belt ply 7B isset to 0.95 to 1.10,

[0038] (2) in the tread thickness K,

[0039] a tread thickness minimum position Qk in which the treadthickness K becomes a minimum value Kmin is provided in the region Y,

[0040] a ratio Kmin/Kc between the minimum value Kmin and a treadthickness Kc at a position of the tire equator C is set to 0.97 to0.998,

[0041] a ratio Kb/Kc between the tread thickness Kb and the treadthickness Kc at a position of an outer end of the second belt ply 7B isset to 1.2 to 1.5.

[0042] At that time, the tread thicknesses T and K are increased fromthe tread thickness minimum positions Qt and Qk to the position of thetire equator C and the position of the second belt ply. The positions ofthe tread thickness minimum positions Qt and Qk may be different but ingenerally the positions are the same.

[0043] Here, there is a correlation between the tread thicknesses T, Kand the grounding surface shape, and by limiting the tread thicknesses Tand K as described above, it is possible to restrain the swellingtendency in the region Y in the grounding shape, to restrain theshortage of the grounding length at the tread grounding end E, toimprove the grounding surface shape, and to equalize the wear.

[0044] If the ratio Tmin/Tc in the tread thickness T is greater than0.97, the effect for restraining the swelling tendency in the region Yis eliminated, and adversely if the ratio Tmin/Tc is smaller than 0.92,the grounding length in the region Y becomes excessively small, andrailway wear is generated in the shoulder groove Gs.

[0045] If the ratio Tb/Tc in the tread thickness T is smaller than 0.95,the grounding length at the tread grounding end E becomes excessivelysmall, the shoulder wear is generated. If the ratio Tb/Tc exceeds 1.10,adversely, the grounding length at the tread grounding end E becomesexcessively great, the wear in the tread center portion Jc is increased,and there is a tendency that a so-called center wear is generated.

[0046] In the tread thickness K as well, if the ratio Kmin/Kc is greaterthan 0.998, the effect for restraining the swelling which is prone to begenerated in the region Y is eliminated, and adversely if the ratioKmin/Kc is smaller than 0.97, the grounding length in the region Y isexcessively small, and railway wear is generated in the shoulder grooveGs. If the ratio Kb/Kc is smaller than 1.2, the grounding length at thetread grounding end E becomes excessively small, the shoulder wear isgenerated. If the ratio Kb/Kc is greater than 1.50, the grounding lengthat the tread grounding end E becomes excessively great, the wear at thetread center portion Jc is increased, and there is a tendency that aso-called center wear is generated.

[0047] When the second belt ply 7B is of a single projecting arc havinga radius of curvature Rb of 450 to 700 mm, the grounding surface shapeis improved by the tread thicknesses T and K. When the radius ofcurvature Rb of the second belt ply 7B is out from the above range, andwhen the second belt ply 7B is formed with a curved line other than thesingle arc, the correlation between the grounding surface shape and thetread thicknesses T, K becomes weak, the wear is not equalizedsufficiently. In order to obtain the distribution of the treadthicknesses T, K, in this embodiment, the tread outline S in the treadcenter portion Jc is formed of a projecting arc outline S1 using asingle arc or a plurality of arcs, and the tread outline S in the treadshoulder portion Js is formed of a substantially straight outline S2.

[0048] Although the preferred embodiment of the present invention hasbeen described in detail above, the invention is not limited to theillustrated embodiment, and the invention can variously be modified andcarried out.

Embodiment

[0049] Heavy duty tires having a structure shown in FIG. 1 and a size of295/80R22.5 were prototyped based on specifications shown in Table 1,and uneven wear performance of the prototyped tires were tested, and theresults are shown in Table 1.

[0050] The grounding length shown in Table 1 is a length in thecircumferential direction measured in the grounding surface shape when astandard load was applied to the tire in the standard internal pressurestate in which the tire was assembled into the standard rim and astandard internal pressure was charged into the tire. A grounding lengthat the tire equator is L1, a grounding length of the side edge of thegroove on the side of the tire equator C of the shoulder groove Gs (inthis example, a distance of 60 mm from the tire equator) is L2, agrounding length of the side line of the groove on the side of thegrounding end of the shoulder groove Gs (in this example, a distance of76 mm from the tire equator) is L3, and a grounding length at thegrounding end E is L4. If a variation in the grounding length issmaller, the uniformity of wear is higher and more excellent.

[0051] (1) Wear Property;

[0052] The prototyped tires were mounted rims (22.5×8.25; having a rimwidth narrower than a standard rim by one rank) of all wheels of a truck(2-2•D type) under an internal pressure (900 kPa), and the vehicle wasallowed to run through a distance of 100,000 km, and wear amounts atvarious positions were measured after running. A wear amount at the tireequator is defined as Z1, a wear amount at the side line of the grooveon the side of tire equator C of the shoulder groove Gs (in thisexample, a distance of 60 mm from the tire equator) is defined as Z2, awear amount at the side line of groove on the side of the grounding endof the shoulder groove Gs (in this example, a distance of 76 mm from thetire equator) is defined as Z3, and a wear amount at the grounding end Eis defined as Z4. The results are evaluated based on an index in which acomparative example 1 is 100. As the value is smaller, the wear amountis smaller. TABLE 1 Embodiment Embodiment Embodiment EmbodimentComparative Comparative 1 2 3 4 example 1 example 2 Tread groundingwidth WT <mm> 240 240 240 240 228 240 Belt ply width WB <mm> 220 220 220220 204 220 Radius Rb of belt ply <mm> 580 580 580 580 580 580 Groovewidth of shoulder groove (*1) 13 13 13 13 13 13 <mm> Tread thickness T<mm> Tmin (*2) 22.6 23.3 22.6 22.6 24.0 21.8 Tc 24 24 24 24 24 24 Tb24.2 24.2 24.2 25.7 24.0 25.0 (Ratio Tmin/Tc) 0.94 0.97 0.94 0.94 1 0.91(Ratio Tb/Tc) 1.01 1.01 1.01 1.07 1 1.04 Tread thickness K <mm> Kmin(*2) 27.5 27.7 27.1 27.5 28.1 26.4 Kc 27.8 27.8 27.8 27.8 27.8 27.8 Kb37.0 37.0 37.0 41.0 33.1 37.0 (Ratio Kmin/Kc) 0.99 0.995 0.975 0.99 1.010.95 (Ratio Kb/Kc) 1.33 1.33 1.33 1.48 1.19 1.33 Ratio of groundinglength L2/L1 0.95 0.97 0.95 0.95 0.94 0.92 L3/L1 0.94 0.96 0.93 0.930.93 0.91 L4/L1 0.92 0.92 0.91 0.95 0.89 0.92 Wear property Wear amountZ1 100 100 100 100 100 100 Wear amount Z2 97 95 100 100 100 105 Wearamount Z3 95 92 99 92 100 110 Wear amount Z4 85 85 88 80 100 95

[0053] As shown in Table, it could be confirmed that in the tire of theembodiment, the uneven wear was improved.

INDUSTRIAL APPLICABILITY

[0054] As described above, according to a tire of the present invention,it is possible to improve the grounding surface shape by limiting thetread thickness from the second belt ply and the tread thickness fromthe carcass, and it is possible to enhance the uneven wear resistanceperformance and to equalize the wear.

1. A heavy duty tire comprising a carcass extending from a tread portionto a bead core of a bead portion through a sidewall portion, and a beltlayer having a first belt ply arranged inside the tread portion andoutside the carcass and a second belt ply outside the first belt ply,wherein said second belt ply is curved in a form of projection of asingle arc having a radius of curvature Rb of 450 to 700 mm, when atread thickness between an outline of a tread surface and said secondbelt ply is defined as T, a tread thickness minimum position Qt in whichsaid tread thickness becomes a minimum value Tmin is provided in aregion Y which is away from a tire equator C by a distance of 0.5 to 0.7times of a tread grounding half width, a ratio Tmin/Tc between theminimum value Tmin and a tread thickness Tc at the position of said tireequator C is 0.92 to 0.97, and a ratio Tb/Tc between a tread thicknessTb at a position of an outer end of said second belt ply and said treadthickness Tc is 0.95 to 1.10, when a tread thickness between saidoutline of the tread surface and said carcass is defined as K, a treadthickness minimum position Qk in which said tread thickness K becomes aminimum value Kmin is provided in said region Y, a ratio Kmin/Kc betweenthe minimum value Kmin and a tread thickness Kc at a position of saidtire equator C is 0.97 to 0.998, and a ratio Kb/Kc between the treadthickness Kb at a position of the outer end of said second belt ply andsaid tread thickness Kc is 1.2 to 1.5.
 2. The heavy duty tire accordingto claim 1, wherein said tread thicknesses T and K are increased fromtread thickness minimum positions Qt, Qk to a position of said tireequator C and a position of the outer end of said second belt ply. 3.The heavy duty tire according to claim 1 or 2, said tread portionincludes two or more circumferential main grooves which continuouslyextend in the circumferential direction, a groove center line of ashoulder groove disposed on the outermost side in the tire axialdirection of the circumferential main grooves passes through said regionY.
 4. The heavy duty tire according to any one of claims 1 to 3, whereina belt cord of said belt layer and a carcass cord of said carcass aremetal cords.